Criteria for the choice of a monitoring equipment

The user of a monitor unit for the detection of radioactive nuclides in scrap has to study the conditions for his special installation. It is very important to verify the way and the transport units for the scrap. For example, scrap delivery by rail shows little variation of transport unit size, way and position of the wagon with the disadvantage of big scrap cargoes. Lorries have a large variation in size and it is very difficult to have good measuring condition (position, speed etc.). Only the size is normally a little bit smaller than by rail. The next is to find the best location on the site for the logistic and measuring conditions. It must be assured that all the delivered scrap passed by this point with a documentation. The optimum of detection and verification of the measurements has to be prepared. The criteria for the sensitivity of the detectors to the ionising radiation must be found. In our company we declare a shielded radioactive Cs-137 source with a high activity, normally used in radiometric measuring units, having a dose rate at the surface near 7.5 pSv/h. This metal bloc has to be found in all positions of the scrap cargo. The other criteria for the choice of the monitor: unit are the maintenance, the control of function, sensitivity and price. The first installation of the monitor unit has to be controlled by a specialist with tests. It is also recommended to install a documentation of all measured cargoes. With this documentation it will be assured that all cargoes are controlled, and it is easier to clarify the reasons of malfunction like vibration, other sources (ceramic, radiometric sources, etc.). On our site, we installed finally two types of detectors, first in 1995 for the scrap delivered by rail working in the standing position of the wagon on the weight bridge. Therefore a small CsI detector, which is very sensitive to gamma rays, was installed with the best positions around the scrap-wagon. Second, for the delivery by road through only one entry to our plant, used big plastic detectors which also assured a good result of detection in a slow motion of the scrap. (author)